1. Field of the Invention
The present invention relates to a method for removing harmful heavy metals, from various types of sludge, which prevent effective use or safe dumping of the sludge.
2. Description of the Related Art
It is known that sewage sludge, sludge from human waste, and sludge from food plants contain harmful heavy metals. Such organic sludge can be used as, for example, compost by fermentation. It is not easy to remove the heavy metals from the sludge before or after fermentation.
The sludge is generated in a large quantity and, today, is thrown away as it is or incinerated. The heavy metals contained in the sludge or ash generated as a result of incineration may possibly contaminate the dump site, or vaporize and scatter into the air and cause environmental pollution. Accordingly, it is an urgent and important task to develop a method for efficiently rendering the sludge non-harmful.
Various methods have been developed for removing heavy metals from the sludge.
For example, Scott et al. have attempted removing heavy metals using various acids, and reported that zinc, copper, iron and the like can be removed by adding sulfuric acid to sewage sludge in water so that the resultant mixture has a pH of about 1.5 and then boiling the mixture (Environ. Sci. Technol., 9(9), 849-855 (1975)). This method was studied later for industrial use as "hot-acid treatment", but was not put into practice.
Oliver et al. have reported a method of oxidizing sewage sludge by chlorine and described that zinc, nickel and the like can be removed by hydrochloric acid (pH 3.5) generated in the process of this method (J. WPCF, 47(10), 2490-2497 (1975)).
Oliver. et al. have reported that heavy metals contained in sewage sludge, for example, cadmium, chromium, copper, iron, lead, zinc and nickel can be removed using a sulfuric acid or hydrochloric acid solution of pH 1.5 at room temperature (Water Res., 10, 1077-1081 (1976)). This method can remove nickel at a relatively high ratio of 80% or more, but can remove other materials at a ratio of only about 50%. Specifically, copper is removed only at a ratio of 18% either by the sulfuric acid or hydrochloric acid solution.
Japanese Laid-Open Patent Publication No. 55-8885 discloses that lead, zinc and cadmium in sewage sludge can be removed using a mixture (pH 1-1.5) of hydrochloric acid and an aqueous solution of hydrogen peroxide. However, copper and nickel are removed at a relatively low ratio of about 50 to 70% by this method.
Satoh et al. have reported a method of treating digested sewage sludge by using an aqueous solution of hydrogen peroxide and then adding an acid, and also a method of treating the digested sewage sludge by an acid while conducting aeration (Kankyo Gijutsu, 15(10), 813-817 (1986)). By the method using an aqueous solution of hydrogen peroxide and acid, the acid is added so that the final concentration of the acid in a mixture of the treating solution and the digested sewage sludge is 0.5 N. Usable acids include nitric acid, hydrochloric acid, sulfuric acid, and phosphoric acid. It has been reported that when nitric acid is used, copper, zinc, cadmium and lead can be removed at a ratio of 80% or more. When phosphoric acid (phosphoric acid content: about 1.6% wt/vol) is used, the removing ratios of most heavy metals are low as follows: copper: 16.6%; zinc: 56.9%; cadmium: 27.5%; and lead: 3.2%.
Nakayama et al. have studied a method for removing heavy metals using bacterial leaching, and Hashimoto et al. have showed that the heavy metals are dissolved by sulfuric acid generated by sulfur bacteria (Mizushori Gijutsu, 28(5), 13-28 (1987)). Accordingly, the method of Hashimoto et al. is basically the same as the methods using sulfuric acid.
Japanese Laid-Open Patent Publication No. 1-104400 describes that mercury, cadmium and lead are removed by treating the sludge generated by the process of treating waste water obtained in the process of incinerating garbage from households or dumped refuse, using hypochlorous acid or hypochlorite and then using a mineral acid.
As described above, various methods have been developed and studied, but all these methods have drawbacks that use of dangerous acids such as sulfuric acid, nitric acid, and hypochlorous acid is required or it is difficult to remove the heavy metals efficiently. Accordingly, none of the above-described methods have been put into practice.